With reference to FIG. 21, the configuration of a conventional plasma processing apparatus will be described. This plasma processing apparatus includes a chamber 1 and also includes an antenna section 3 which covers the upper side of the chamber that opens, and serves as high frequency supply means. Antenna section 3 includes an antenna cover 3a made of aluminum alloy, a retardation plate 3b made of ceramics and an antenna plate 3c made of copper alloy. Antenna plate 3c is provided with slots 20 which are a plurality of long through holes. In addition, a top plate 15 made of a dielectric material such as, for example, quartz or ceramics is placed between antenna section 3 and chamber 1. Here, the “top plate” is referred to as “dielectric window,” “microwave transmitting window” or the like in some cases. Top plate 15 is secured to chamber 1 by a top plate presser ring 16. Antenna section 3 is secured by an antenna section retainer ring 17.
A susceptor 7 is placed in chamber 1 and, when plasma processing is carried out, exhausting the inside of chamber 1 by means of a vacuum pump 9 and a reactive gas is introduced from a gas inlet (not shown) after a substrate 11 that is to be processed is placed on the upper surface of susceptor 7. High frequency waves are generated by a high frequency generator 5. These high frequency waves are transmitted to antenna section 3 through a waveguide 6, passes through retardation plate 3b, is distributed in a constant range by means of the plurality of slots 20 in antenna plate 3c, and is supplied toward chamber 1. The high frequency waves pass through top plate 15 and convert the reactive gas into plasma. As a result of this, plasma 13 is produced in chamber 1 and plasma processing is carried out on substrate 11. Here, although waveguide 6 is a coaxial formed of an inner side conductor 6a and an outer side conductor 6b in this example, there may be waveguides in other forms.
In the state where the density of plasma 13 that has been produced in chamber 1 is increased to the degree where the cut-off frequency by the plasma becomes higher than the high frequency waves, the higher the density of the plasma is the greater the ratio of reflection of the high frequency waves from the interface between top plate 15 and plasma 13 becomes at the time when the high frequency waves are supplied into chamber 1. In the case where top plate 15 is thinner than a certain degree, the high frequency waves that have been reflected from the interface returns to high frequency generator 5 along waveguide 6 from top plate 15 and is again reflected toward antenna section 3 from a matching unit (not shown) which is generally placed between antenna section 3 and high frequency generator 5. As a result of this, the electric magnetic field becomes very strong in waveguide 6 between antenna section 3 and the matching unit, causing an abnormal discharge and a power loss.
On the other hand, in the case where top plate 15 is thicker than a certain degree, the reflected high frequency waves do not return along waveguide 6 but rather, tends to repeat the reflections from the outer surface of top plate 15 so as to be confined within top plate 15 forming standing waves. In the case where such standing waves occur, as shown in FIG. 22, intense electric field regions 18 locally appear within top plate 15 as viewed in the radius direction of top plate 15. Here, FIG. 22 shows only the left half. The arrows in FIG. 22 indicate the directions of propagation of the high frequency waves. In this case, the stronger electrical fields are generated in the vicinity of the center of top plate 15. As a result of this, such effects are reflected within chamber 1. FIG. 23 shows the plasma density distribution within chamber 1 at this time. That is to say, the plasma density is high in the vicinity of the center within chamber 1, impairing the uniformity of the plasma density. In the case where the uniformity of the plasma is impaired, the uniformity of plasma processing is also impaired.
In addition, the smaller the distance between slots 20 and plasma 13 is, that is to say, the thinner top plate 15 is, the stronger the electrical field generated in the vicinity of the surface of plasma 13 by the electrical field formed in slots 20 becomes. In other words, the efficiency of power supply to the plasma is increased. Though this is preferable, top plate 15 is made of a dielectric material and a material such as quartz or ceramics is actually used; therefore, weakness in the physical strength becomes a problem limiting the reduction of the thickness of the top plate. In the case where quartz is used for the top plate in a plasma processing apparatus for carrying out plasma processing on a semiconductor wafer having a diameter of 300 mm, for example, the thickness of the top plate must be at least 40 mm from the point of view of ensuring the physical strength and such a thickness easily allows standing waves to occur inside the top plate. Undesired standing waves occurring inside the top plate reduces the efficiency of power supply, impairs the uniformity of the plasma density within the chamber and impairs the uniformity of plasma processing.